Melo&iracece 



275 



small spines or teeth. The girdle usually exhibits a well-marked 

 structure. The chromatophores are small, numerous, parietal and 

 plate-like. 



Genus Melosira Ag., 1824. [Gaillionella Bory ; Lysigonium 

 Link; Liparogyra Ehrenb. ; Orthosira Thwaites; Sphcerophora 

 Hass.] The frustules are cylindrical, ellipsoidal, or globular, and 



B 





Fig. 126. A and B, Melosira arenaria Moore, from Shipley Glen, W. Yorks. 

 C E, M. varians Ag., from the river Cam at Cambridge; E, showing forma- 

 tion of auxospore. (All x 450.) 



are united to form filaments of variable length. The valve-view is 

 circular and plainly punctate. Auxospores are formed without 

 conjugation, by the rejuvenescence of the contents of a mother-cell 

 to form a much larger daughter-cell, the long axis of which is either 

 parallel or at right angles to that of the mother-cell. These large 

 daughter-cells continue to divide while still remaining parts of the 

 original filament. 



The genus is divided into four sections: Sect. 1. Eumelosira Schiitt, 

 including M. arenaria Moore, J/. granulata (Ehrenb.) Ralfs and M. Roeseana 

 Rabeuh. ; Sect. 2. Lysigonium Link, including J/. carians Ag. ; Sect. 3. 

 Podosira Ehrenb., marine ; Sect. 4. Gaillionella Bory, including M. numtnu- 

 loides (Bory) Ag. 



There are some six or eight freshwater .species occurring in the British 

 Islands. J/. rarians Ag. is one of the most abundant of the centric Diatoms, oc- 

 curring in large quantities in ponds, ditches and slow rivers (fig. 126 C E). J/. 

 arenaria Moore occurs on wet rocks, sometimes forming crisp mat-like masses 

 on dripping sandstone rocks. It is common on the Carboniferous Sandstone 

 of England. J/. granulata (Ehrenb.) Ealfs occurs in boggy pools and also in 



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